1. Field of the Invention
The present invention relates to a dual armature device for transmitting the movement to fans for cooling the coolant in motor vehicles and for coupling and decoupling an actuator to the fans for controlling the speed of the fans.
2. Description of Related Art
It is known in the technical sector relating to the cooling of coolants contained in motor-vehicle radiators that there exists the need to force air onto the radiator in order to obtain more rapid dissipation of heat from the coolant to the exterior, said forced air flow being obtained by causing rotation of a fan which is normally mounted on the shaft of the water pump or on the driving shaft or on a driven and fixed shaft carrying a pulley which receives movement from a belt actuated by the driving shaft.
It is also known that said fan must be made to rotate only upon reaching a certain predefined temperature of the water detected by means of a thermostat which activates an electromagnetic clutch, closing of which causes the fan to start rotating.
More particularly it is required that a motor vehicle fan must be able to rotate:                at a lower speed than that of the transmission shaft for cooling in low external temperature conditions;        at a speed equal to or even greater than that of the transmission shaft in the case of higher external temperatures or use in severe conditions which cause overheating of the engine;        at zero speed, namely with the fan which does not rotate at all and remains in an idle condition with respect to the transmission shaft, in the case of particularly low temperatures at which further cooling is of no use or even damaging.        
In an attempt to achieve these performance features, coupling systems of the mixed type with electromagnetically operated friction clutches and drive couplings based on the use of parasitic currents generated by rotation of a conducting element in the vicinity of permanent magnets have been developed.
DE-32 03 143 describes, for example, an arrangement in which the driving shaft is connected to the rotor of an electromagnetic clutch, which is engaged by an armature connected to the fan for direct driving, whereas low speed conditions make use of the engagement between a conducting disk, rotating with the transmission shaft, and the permanent magnets integral with the fan, said engagement causing transmission of movement at a low speed as a result of relative slipping between the two parts. With this solution, however, it is not possible to obtain the idle condition of the fan.
In addition, the known devices do not envisage the possibility of maintaining an albeit slow rotation of the fan (fail safe mode) in the event of breakage and/or complete interruption of the power supply to the coils of the clutches as occurs for example in the case of total electrical failure.